Free Excretion MCQs with Answers

1. The chief nitrogenous waste product of humans is

• A. ammonia
• B. urea
• C. uric acid
• D. creatine

Explanation: Surplus amino acids are deaminated in the liver and the toxic ammonia produced is converted at once into far less toxic urea by the ornithine cycle, which the blood then carries to the kidney. Ammonia is too poisonous to accumulate and is excreted directly only by animals surrounded by water, such as bony fish. Uric acid is the main waste of birds, reptiles and insects.

2. Birds and insects excrete uric acid rather than urea mainly because

• A. uric acid is more toxic and so must be removed faster
• B. they have no liver in which to make urea
• C. uric acid is almost insoluble and can be excreted as a paste, saving water
• D. uric acid contains no nitrogen

Explanation: Being nearly insoluble, uric acid can be voided as a semi solid with very little water, which suits animals that must conserve water and, in birds and reptiles, allows waste to be stored safely inside a shelled egg. Uric acid is in fact the least toxic of the three common nitrogenous wastes. Making it costs more energy than making urea, which is the trade off for saving water.

3. In a kidney dialysis machine, waste is removed from the patient's blood because

• A. the dialysis fluid contains enzymes that destroy urea
• B. the blood is heated so that urea evaporates
• C. urea is actively pumped across the membrane
• D. the dialysis fluid contains no urea, so urea diffuses out across a partially permeable membrane

Explanation: Dialysis fluid is made with the same concentrations of glucose and mineral ions as normal blood plasma but with no urea at all, so only the wastes follow a concentration gradient out while useful substances have no gradient and stay. The fluid flows in the opposite direction to the blood, keeping the gradient steep along the whole membrane. Nothing is pumped, since the whole process is diffusion.

4. Which of the following is an example of excretion rather than egestion?

• A. The passing of undigested cellulose in faeces
• B. The loss of dead cells from the surface of the skin
• C. The removal of hair when it is cut
• D. The removal of carbon dioxide by the lungs

Explanation: Excretion is the removal of the waste products of the body's own metabolism, and carbon dioxide is made by respiration inside every cell, so the lungs are excretory organs. Faeces consist largely of material that was never absorbed into the cells at all, so removing them is egestion. This distinction is a standard exam point and turns on whether the substance was ever part of the body's metabolism.

5. Aldosterone increases

• A. Water excretion
• B. Sodium reabsorption
• C. Potassium reabsorption
• D. Glucose excretion

Explanation: Aldosterone increases Na+ reabsorption and K+ secretion in the distal tubule and collecting duct.

6. Which of the following organisms eliminate nitrogenous waste mainly in the form of uric acid?

• A. Freshwater Fish
• B. Amphibians
• C. Birds
• D. Mammals

Explanation: Uric acid is almost insoluble, so it can be excreted as a paste with very little water, which suits birds both for flight and for developing inside a shelled egg where waste must be stored safely. Freshwater fish excrete ammonia directly into the surrounding water, and mammals and adult amphibians make urea. The choice of waste product tracks how much water the animal can afford to lose.